15min:

ERIN N. SHARP-WILLIAMS, MELANIE A. ROBERTS, DAVID J. NESBITT, JILA, National Institute of Standards and Technology and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309; ROBERT F. CURL, Department of Chemistry and Rice Quantum Institute, Rice University, Houston, Texas 77005.

The ethynyl radical, C₂H, is an important reactive intermediate both in combustion processes, as it is readily formed in an acetylene (C₂H₂) flame, and in the chemistry of the interstellar medium, where it is suspected to be involved in the formation of longer carbon chain species (C_nH). We have recently interrogated several of the vibronic transitions to the low-lying excited A state from the vibrationless level of the ground electronic state via high resolution infrared spectroscopy. This was done using direct absorption laser spectroscopy in a slit-jet discharge supersonic expansion of C₂H₂ diluted in a Neon/Helium gas mixture. In comparing our spectra with those already published using magnetic rotation spectroscopy^,, we find discrepancies between rovibronic frequencies in the 0₀⁰ band at 3600~cm^-1. The inconsistency is localized in the excited state, by comparison of 2-line combination differences with mm-wave measurements of the ground state by Thaddeus and coworkers. Calculating the ² energy levels using both Hund's case (a) and (b) basis sets and revisiting the analysis in the aforementioned work, we have determined that the discrepancies arise from a parity mislabeling of the lambda-doubled excited states. The improved low J signal intensities and resolution of satellite transitions that are observable under sub-Doppler, jet-cooled conditions complement the previous data and permit refinement of the rotational, spin-rotational, and lambda-doubling constants.